le Châtelier's Principle: Temperature

The effect of temperature on an equilibrium

Increasing temperature favors the production of brown NO₂ in the boiling water on the left. The ice cold container on the right contains more molecules of colorless N₂O₄, so its color is lighter. Dinitrogen tetroxide is an oxidizer, commonly used in spacecraft.

To determine the effect of temperature on a reaction, first write the equation including the heat term in the equation. Remember that a negative

H is written on the products side, and a positive

H with the reactants.

For example, consider this reaction:

2NO₂(g)

N₂O₄ (g)

H = -58 kJ

which we could write as:

2NO₂ (g)

N₂O₄ (g) + 58 kJ

Since le Châtelier's principle is not quantitative, the size of the energy term is not critical. We could simply put:

2NO₂ (g)

N₂O₄ (g) + energy

Now consider what will happen if we increase the temperature.

Click for help on creating a stress table

Use le Châtelier's principle by applying the following three steps:

1.Identify the stress. In this case the applied stress is an increase in temperature

2. Identify how the system will respond to the stress. Remember that the system will always do the opposite of the applied stress. In this case, the system will try to decrease the temperature. This means the energy term must decrease.

Applied Stress	le Chatelier's Principle Prediction of Response to Stress
	2NO₂ (g)	N₂O₄ (g)	+ energy
Increase T			Decrease

3. Identify how each substance will respond to this change, using the "see-saw" effect. In this case each substance on the right of the equilibrium sign will go down, and everything on the left side will go up. The reaction will shift to the left.

Applied Stress	le Chatelier's Principle Prediction of Response to Stress
	2NO₂ (g)	N₂O₄ (g)	+ energy
Increase T	Increase	Decrease	Decrease

Since NO₂ is reddish-brown in color, the container will become much darker brown, as shown in the above image for the reaction at 100 ^oC.

Exactly the opposite will happen if we decrease the temperature.

Use le Châtelier's principle by applying the following three steps:

1.Identify the stress. In this case the applied stress is a decrease in temperature

2. Identify how the system will respond to the stress. Remember that the system will always do the opposite of the applied stress. In this case, the system will try to increase the temperature. This means the energy term must increase.

Applied Stress	le Chatelier's Principle Prediction of Response to Stress
	2NO₂ (g)	N₂O₄ (g)	+ energy
Decrease T			Increase

3. Identify how each substance will respond to this change, using the "see-saw" effect. In this case each substance on the right of the equilibrium sign will go up, and everything on the left side will go down. The reaction will shift to the right.

Applied Stress	le Chatelier's Principle Prediction of Response to Stress
	2NO₂ (g)	N₂O₄ (g)	+ energy
Decrease T	Decrease	Increase	Increase

Since N₂O₄ is colorless, the container will become much lighter, as shown in the above image for the reaction at 0 ^oC.

Check your understanding of le Châtelier's principle with these questions.

A simple experiment to demonstrate the effect of temperature on a reaction.

Halogen lights are a good example of temperature and le Châtelier's principle.